Energy and Material Balances

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Energy use monitoring is basic for an energy efficiency program and its management. Lord Kelvin said ' To measure is to know.' and ' If you cannot measure it, you cannot improve it.' Measurement allows tracking and control, so that it is possible to learn about how much and what source of energy is used, and for which purposes. It is the foundation of any energy efficiency program.

A comprehensive site energy balance must include all energy sources used by refinery units, especially fuel, steam and power in all its forms. Only after a good energy balance has been generated and regularly checked can the system be effectively evaluated. The current status can be determined and a baseline established for energy consumption of the facility. With this tool it will be possible to:

• Track historical energy consumption, learning which production events impact it.

• Forecast energy consumption with production.

• Prioritize points of attention and consequent actions to reduce obvious losses.

• Determine the success of those actions by comparing change from expected and effective consumption.

• Compare with other typical refineries and units to help set realistic targets for improvement.

• Get data to spread energy opportunities and generate effective participation in the program by company staff and commitment from management.

• Together with accounting data, generate costs for each energy source.

• Evaluate impact of investment by demonstrating energy savings with future projects.

To set a good energy balance, the first step is to gather all available data and determine what additional information is needed to estimate energy use. It is necessary to prepare balances for every energy form used, normally including fuels, steam and power.

A reasonably accurate fuel balance is of main importance because a refinery usually relies by more than 80% on fuel consumption. Commonly, fuel is the main direct heating source for the major processes on radiating furnaces, and steam generation is mostly done in boilers using fuels. Because a significant amount of fuel used as energy comes from the refinery's own production, this balance influences the complex throughput and production. These flows must be checked with overall refinery production mass balance before being considered in the energy balance. But some fuel is imported, especially natural gas, and must be included in the balance.

The most difficult part of the energy balance is normally the steam balance. Steam is consumed at different levels of pressure and temperature and they are closely related to each other. Usually the highest pressure level is generated in boilers and the others lower levels are produced by steam turbines and valves. After consumption, steam may become condensate that should be returned whenever possible to the boilers for economic reasons, and the rest of it most certainly will end up in the wastewater system either as drained condensate or as oily water. So a good steam balance should be based on an equipment-by-equipment sum of steam flows for each pressure level. The water balance is a consequence of the steam balance. Since you cannot measure many of the steam uses that end up as losses, measuring the inflow of water is a valuable help to close accounts on the steam balance. Because of a natural tendency for refineries to be attached to other facilities that tend to either consume or produce steam, there is a good chance that imports or exports of it are likely to occur and must be taken into account. Another mass balance appears since steam energy balance is related to water and condensate, and they all together add up in the water consumption balance.

It is not uncommon that a large proportion of power demands in a refinery, meaning electricity, is purchased from an external provider, although for economic reasons, the rest of it is usually generated inside the refinery by means of fuels or steam from waste heat recovery. The power balance tends to be the easiest one among all other energy sources because it is metered directly in energy. The balance is done by comparison of the total power supplies, purchased and self-produced, with the total load, the whole sum of all electrical demands. The tricky part is relating total electrical demand to overall energy demand, because the electrical power rate over steam and fuel will depend on generating system efficiency. This isn ' t a big issue, and monitoring and tracking of energy balance of the utility generating unit will do this job. Nor does it influence the overall energy balance, but it may become an issue when the aim is to share electricity costs among process units.

The energy balance is a regular procedure that must be kept, registered and improved. Remember that this is the main pillar over which the energy efficiency program is erected and maintaining it is a basic daily work. To accomplish it, a lot of data is required, and some standardization of metering, laboratory analysis and basic assumptions are needed.

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